1. Field of the Invention
This invention concerns the control of automotive exhaust particulate emissions, particularly the removal of lead-containing particulates from the exhaust gases.
2. Description of the Prior Art
Exhaust gases of internal combustion engines normally contain small amounts of particulate materials. The particles are either carbonaceous materials from incomplete combustion of fuel or are derived from additives normally used in fuels and lubricants.
Gasolines employed as fuels for internal combustion engines for automotive use usually contain tetraalkyllead compounds such as tetraethyllead, tetramethyllead, mixed alkyl leads and the like to improve the octane rating and provide antiknock characteristics. To minimize deposition of the lead compounds in the combustion chambers, scavenging compounds such as a mixture of ethylenedibromide and ethylenedichloride are used together with the lead antiknock compounds. It is known in the art that hydrogen halides are also present in the exhaust gases. It is believed that during the combustion process relatively volatile lead halides are formed which are carried out of the combustion chambers with the exhaust gases. In the exhaust system, most of the lead halides are condensed to particulate matter. Since the conventional automotive exhaust systems are not designed to remove particulate matter, most of the particulates are exhausted into the atmosphere.
Many suggestions have been made in the art for removing particulate matter and particularly lead particles from the exhaust gas. Prior art devices employing cyclone separators, baffle systems, liquid fluxes, filters, and the like, have generally been unsuccessful in removing particulate matter for a number of reasons including insufficient removal of lead particulates, inefficient removal of small size particles, development of excessive back pressures, or insufficient durability.
In U.S. Pat. No. 3,397,034, an agglomeration zone of particulated alkaline material is provided before an inertial separator. The material in the zone, e.g. bauxite, is selected to be reactive with lead and acid compounds in the raw exhaust gas and to form agglomerates as the result of reaction between the material and compounds. Collection of agglomerated lead is accomplished concurrently with loss of the substrate reaction product upon which the lead has accumulated. Aggregates of the alkaline material reaction product having lead particles adhered thereto sluff off in relatively large pieces and collect in the subsequent solids separation zone. Exhaust gas leaving the separator is in a condition such that it does not destroy the alumina support of a catalyst downstream, which alumina is a kind otherwise destroyed by raw exhaust gas.
In U.S. Pat. No. 3,485,593, is disclosed an agglomeration unit using as agglomeration aids: marble chips, silicon carbide, iron nails, synthetic or natural zeolites, wire mesh and the like. There is no suggestion of using the agglomerating alumina disclosed herein. Rather, after exhaust gases pass through the agglomeration unit and are freed of solids, then, the gas is passed through catalyst supported on a high surface area material such as alumina. The disclosed alumina has been calcined at 700.degree. to 800.degree.C to drive off water and therefore not become plugged with exhaust gas solids.
U.S. Pat. No. 3,815,337 discloses an exhaust system comprising a separate agglomerating unit containing alumina interposed in an engine exhaust pipe between the engine and a muffler trap containing a cyclone-type particulate separator. The patent discloses that uncondensed compounds capable of forming fine particulates such as lead halides are absorbed, thus preventing the formation of fine particulates. It suggests that the alumina absorbent degrades resulting in release of coarse alumina particulates into the particulate separator. Thus this system, like that of U.S. Pat. No. 3,397,034, uses alumina as an expendable material during the recovery of particulates.